So it's an ancient scottish centaur that breathes disease, makes diseases around it worse, doesn't have any skin, and has a spreading area of effect? I tell ya, this scp is pulling itself in a lot of different directions to the point of ridiculousness.

I'd throw the spreading area of effect right out. Inevitability is frightening when it sneaks up on us, not when it's tacked on to the effects to make it seem more dangerous.

The consequence of having so many traits is that you aren't paying enough attention to any of them. Why is the Foundation so disinterested in who it is and how it lives? There's much more detail paid to containment procedures based on the disease thing than to its identity as a centaur, etc. The thing speaks a language we can understand, for crying out loud- why haven't we interviewed it, found out who it is and how it got to be this way? And I get that you're trying to build sympathy by having the containment procedures involve literally rubbing salt in its wounds, but for that to work we have to empathize with the creature- and you don't give us any indication of who it is or why we should care about it.

The disease breath seems totally unrelated to the creature itself, and so doesn't engage the reader. There's no common story or thread- just a strange horse with a man's torso and then an out-of-left-field danger anomaly. In the original folklore, it's a type of elf, and there's a whole mythology of elves as troublemakers. Here, you need to develop your own story. This leads me to my next point, about the danger of adaptations.

You can't just describe an existing mythological creature in technical language and have it be a SCP. You have to make it your own, and tell your own story with it. Why does this thing want to cause droughts and famine? Why does it hate crops? How does it feel about Scotland and how did it come to be captured by the Foundation? Make the Nukelavee a character we're interested in. Show some side of its personality that we wouldn't get from that wikipedia article. Otherwise all you've got is an entry in the Monster Manual.

Don't try so hard to sound technical- the first sentence in the description is laughable. You can refer to it as a horse and give its species name if you desire, but what you've got is silly. Also, make clear whether it's two organisms or one organism with two heads- I'm getting the image of a regular horse, without skin, and with half a man sticking out the top, correct? If not, please state more directly what bits it has where. And avoid the struckthrough text effect, it's hard on the eyes and looks sloppy.